On fine structure of nascent UHMWPE reactor powders

Reactor powder of a number of commercial ultra-high molecular weight polyethylenes synthesized on heterogeneous Ziegler-Natta catalyst in a conventional slurry process have been investigated with the help of scanning electron microscopy (SEM), wide-angle x-ray scattering (WAXS), differential scanning calorimetry (DSC), and low-frequency Raman spectroscopy. The SEM study reveals a complicated structure of nascent particles consisting of a small (0.5 - 1.0 mum) spongy-like fibrillar spheres. It is suggested that the elementary morphological units are fibrils formed by the small defective crystallites with the dominating crystalline distortions caused by microstrains. The comparison of longitudinal crystallite sizes derived from WAXS data with the straight chain segment length calculated from Raman shift frequency and the length of extended segments computed from the true melting interval measured by DSC allows to conclude that there are a large number of extended tie chains bridging the neighboring crystallites, which is not in agreement with a lamellar model. The extended chains passing through the noncrystalline regions stabilize a structure and provides its thermal stability. A possible tilting of molecules in the crystals, the location of defects and the distribution of crystallite sizes are discussed.